Chain Motion in an Unentangled Polyethylene Melt: A Critical Test of the Rouse Model by Molecular Dynamics Simulations and Neutron Spin Echo Spectroscopy

Abstract

We have investigated the dynamic structure factor for single-chain relaxation in a polyethylene melt by means of molecular dynamics simulations and neutron spin echo spectroscopy. After accounting for a 20% difference in the chain self-diffusion coefficient between simulation and experiment we find a perfect quantitative agreement of the intermediate dynamic structure factor over the whole range of momentum transfer studied. Based on this quantitative agreement one can test the experimental results for deviations from standard Rouse behavior reported so far for only computer simulations of polymer melt dynamics. [S0031-9007(98)05363-0] PACS numbers: 61.25.HqThe dynamics of polymer chains in a dense melt could be supposed to pose a theoretical problem requiring a very complex and mathematically involved description. We have to describe a liquid of intertwined threads where each of them has on average excluded volume interactions with p N other threads, where N is the degree of polymerization of the chains. According to all experimental evidence so far, e.g., Refs. [1-3], however, it seems that all these complex topological interactions can be completely neglected as long as the degree of polymerization of the chains is below some critical value, the so-called entanglement molecular weight N e . For chains longer than N e the entanglements have to be taken into account [4-7] but for shorter chains the simple Rouse theory [8] is supposed to describe the chain dynamics. Computer simulations of abstract [9,10] as well as atomistic [11] polymer models, on the other hand, show systematic deviations from the Rouse behavior, which can be traced to the interactions between the chains in the melt.We will show in this paper the first detailed quantitative comparison between a molecular dynamics (MD) simulation of the melt dynamics of an atomistic polymer model and a neutron spin echo (NSE) determination of the single-chain dynamics in the same polymer melt. By establishing the quantitative agreement between simulation and experiment for the internal dynamics of the chains we can then draw conclusions about the validity or shortcom-ings of the Rouse model from the combined information of simulation and experiment.Simulations and experiments were performed on a dense polyethylene melt of n-C 100 H 202 at 509 K. Experimentally we had already obtained information on the dynamic behavior of longer chain polyethylene (PE) samples at the same temperature from neutron scattering studies [1,2], and we had validated a united atom (UA) model (CH 2 groups treated as one superatom) [12] as well as an explicit atom (EA) model [13] by simulations of shorter chain alkanes. The C 100 chains are slightly shorter than the entanglement length of PE at this temperature (N e 136 [2]) and long enough to show Gaussian chain statistics in their conformations [14], thereby making them the ideal test system for a description by the Rouse model. After equilibration for 3 ns we performed a NVT (constant number of particles, volume, and temperature) mole...

Zero-shear stress relaxation and long time dynamics of a linear polyethylene melt: A test of Rouse theory

“…The dynamic structure factor of a single chain can be calculated by noting that the random displacements of the normal coordinates are Gaussianly distributed. 2 After some straightforward calculation we find S͑q,t ͒ϭ 1…”

“…Several other studies have been reported on simulations of melts of polymers of comparable size. [1][2][3][4][5][6][7][8][9] They all suggest that individual properties of these melts can well be described by the Rouse model. 10 In this paper we will test whether the Rouse model can be used to predict the time correlation functions of a comprehensive set of physical processes by using one single set of parameters.…”

“…1,2 They found that translational and rotational diffusion of the chains can consistently be described by the Rouse model, but that systematic deviations show up in the internal dynamics of the chains. Also, subdiffusive behavior was observed in the chain dynamics, not predicted by the Rouse model.…”

Chain dynamics of poly(ethylene-alt-propylene) melts by means of coarse-grained simulations based on atomistic molecular dynamics

“…Such simulations have been performed only for medium length chains where large-scale dynamics and rheological properties seem to be well described in terms of the simple Rouse model. [5][6][7][8][9] In order to simulate dynamical and rheological behavior of long chains, one has to resort to coarse-grained models. Many different coarse-grained models have been reported in the literature, 10 being perhaps the most popular one that of bead-spring type, where the polymer chain is represented as a sequence of beads connected by springs.…”

Molecular dynamics insight to phase transition in n-alkanes with carbon nanofillers

“…These values are in good agreement with those reported in the literature, as obtained from experimental evaluation. 46 The self-diffusion coefficient increases with increase in temperature and also with increase in the concentration of CNTs.…”